Neutron Stars and Pulsars

Neutron Stars: Properties and Formation

  • Electron Degeneracy Limit:
    • The electron degeneracy limit is approximately 1.4 times the mass of the Sun.
    • Core collapse might result in some mass loss, but neutron stars should generally not be below this limit.
  • Magnetic Fields:
    • Neutron stars possess extremely intense magnetic fields, over a trillion times stronger than Earth's magnetic field.
  • Temperature:
    • Formed as the core of high-mass stars, originally around 10,000,000,000 degrees Fahrenheit.
    • Cool significantly over time but remain very hot and radiate energy.
  • Luminosity and Magnitude:
    • Low surface area leads to very low luminosities and absolute magnitudes.
  • Radio Energy Emission:
    • Particles accelerate along magnetic field lines, particularly at the north and south magnetic poles.
    • This acceleration results in intense radio energy emission.
  • Focused Beams:
    • The magnetic field focuses radio energy into intense beams along each magnetic axis.

Pulsars: The Rotating Neutron Stars

  • Rotation and Magnetic Axis Alignment:
    • There's no physical law requiring the rotational axis and magnetic axis to be parallel. Generally, they are not aligned (e.g., Earth, Jupiter, Sun).
  • Pulsar Phenomenon:
    • As a neutron star rotates, the beam of radiation sweeps a circle in the sky.
    • If Earth lies in the path of this beam, we observe brief, regular pulses of radio energy.
    • This gives rise to pulsars.
  • Discovery:
    • The first pulsar was discovered in 1965 by Jocelyn Bell Burnell at the Mullard Radio Astronomy Observatory in England.
    • The precise timing of the pulses was initially perplexing.

Neutron Stars in Binary Systems

  • Accretion Disks:
    • Neutron stars in binary systems can pull gas from their companion stars forming an accretion disk.
    • Gas in the disk heats up as it spirals inward.
    • It becomes ionized, and the magnetic field channels the gas onto the neutron star's surface.

Mass Range and Formation

  • Mass Range:
    • Observed neutron star masses range from 1.1 to 2.4 times the mass of the Sun.
  • Formation:
    • Neutron stars represent the final evolutionary stage for high-mass stars with masses below 20 times the mass of the Sun.

End States and Black Hole Formation

  • Summary of Stellar Evolution:
    • High mass stars exhaust core fuel and collapse to form black holes.
  • Neutron Degeneracy and Mass Limit:
    • Neutron degeneracy supports neutron stars, similar to electron degeneracy.
    • There's a maximum speed for neutrons, leading to a maximum mass limit.
    • Most massive observed neutron star is about 2.4 solar masses.
    • Theoretical limit around 3 solar masses.
  • Collapse to Black Hole:
    • If a neutron star exceeds its mass limit (around 3 solar masses), gravity overcomes degeneracy pressure, causing collapse into a black hole.
  • Gravity as Strongest Force:
    • During the collapse to a black hole, gravity becomes the strongest force in nature.